The Jellybeanium Lab: Isotopes and Average Atomic Mass C11-3-01
Thanks to Harvey Peltz & RETSD
Name_____________________
Introduction:
Within any sample of an element there are millions upon millions of atoms. There are
also likely to be various isotopes for this element. That is, although all of the atoms may
be for the element magnesium with 12 protons and electrons, the sample may have
atoms with 12, 13, 14 or more neutrons. Thus, although the atoms all have the same
atomic number (e.g. 12 protons for magnesium atoms), the atoms may have variable
atomic masses (e.g., 12 protons + 12 neutrons for an atomic mass of 24 or 12 protons
+ 13 neutrons for an atomic mass of 25). Because the neutron is such a significant
atomic mass unit and can cause such variability in atomic masses, we need to know
what amount as a percent of the sample is what type of isotope. The activity that follows
will help you to understand this rather complex idea.
Problem:
You are given a sample containing three different colors of jelly beans. Each color of jelly bean
represents an atom for an isotope of the element Jellybeanium (Jm). That is, each jelly bean
type has the same atomic number; but varies in atomic mass because it contains a variable
number of neutrons.
Your job is to determine:
-the relative abundance (%) of each isotope
-the average mass of each bean
-the average atomic mass, in grams, of the element Jellybeanium.
Materials and Equipment:
-Mixture of jelly beans (isotopes), all of the element Jm
-Electronic Balance
Procedure:
1. Spread out your Jellybeanium sample in front of you. Note that there are three Jm
isotopes. Note that the three differ in size and mass. If these were real atoms of three
isotopes of the same element, would the three atom types differ in BOTH size and mass?
Explain.
2. Look at the jelly beans (atoms) of one bean type (isotope). Are they all the same size and
mass? Would they be in a real sample? Explain.
3. Select one jelly bean from the entire sample that you think is the average mass
bean. Determine its mass using the electronic balance. _______ g
4. At this point you could actually determine if this actually is the average mass jelly bean.
How would you do this?
5. Can you see this would be quite the task? Wouldn’t it be even a more impossible task for
a real sample where there were millions upon millions of atoms?
6. Count the number of each type of jelly bean (isotope) in your sample and record in the
appropriate data table. Remember each jelly bean type is the same Jellybeanium
element – just different isotopes varying because of different atomic mass.
7. Determine the average mass of each type of jelly bean and record this in the chart.
8. Calculate the average atomic mass, in grams, of your sample of Jellybeanium.
What method will you use?
Data:
(6 marks)
1. What is the total number of jelly beans?
Table 1: Jellybeanium Type (Isotope) 1:
Mass of all this type of jelly bean in sample
Number of jelly beans in this bean type sample
Average mass of one jelly bean
Percentage abundance of entire sample with
this isotope
Table 2: Jellybeanium Type (Isotope) 2:
Mass of all this type of jelly bean in sample
Number of jelly beans of this type sample
Average mass of one jelly bean
Percentage abundance of entire sample with
this isotope
Table 3: Jellybeanium Type (Isotope) 3:
Mass of all this type of jelly bean in sample
Number of jelly beans of this bean type of
sample
Average mass of one jelly bean
Percentage abundance of entire sample with
this isotope
Calculations and Questions:
1. Percent abundance of each Jellybeanium “isotope” in sample (From table above)
___________________Isotope 1
___________________Isotope 2
___________________Isotope 3
2. Relative (Average) atomic mass of the element Jellybeanium
___________________
You need to check the % abundance AND average mass of each Jellybeanium isotope in
performing this calculation
3. Compare your answers to another group’s. Do they vary? Explain why.
4. Compare this average atomic mass to the predicted average atomic mass point three
on page one. How close were you to the predicted average?
5. In this sample there was considerable variability in the mass of each jelly bean type. That
is all of the same type of jelly bean did not have the same mass. In a sample of an
element would all of the atoms for one isotope have the same mass or would there be
considerable variability like in each bean type? Why or why not.
6. In your own words explain the meaning of the terms:
a) isotope:
b) relative (average) atomic mass of an element:
7. A sample of Magnesium has three isotopes in the following percentages: 12.4% of Mg
24, 46.2 % of Mg 25 and 41.4 % of Mg 26.
a) how do these three isotopes differ?
b) Draw Bohr model diagrams of each isotope including number of protons,
neutrons and electrons.
c) Finally, calculate the relative atomic mass of Mg for this sample.
8.
A sample of Aluminum has three isotopes in the following percentages: 22.6% of
Al 26, 36.2 % of Al 27 and 41.2 % of Al 28. Calculate the relative atomic mass of
Mg for this sample.
It is important to note that for our studies in chemistry the relative atomic mass provided
by the Periodic Table is used in determining the atomic mass of individual atoms and
the molecular mass of compounds. Although the atomic mass unit provides us an
approximate atomic and molecular mass, it fails to take into account the presence of
isotopes in a sample. Thus, when calculating the atomic or molar mass of a
substance, use the relative atomic masses provided on the Periodic Table rather
than adding the number of neutrons and protons in the atoms.